mci: Fix bug if probe parameter is omitted

barebox@Phytec phyCORE-i.MX27:/ mci0.probe=
unable to handle NULL pointer dereference at address 0x00000000
pc : [<a7f24700>]    lr : [<a7f193f4>]
sp : a7fff8e0  ip : 00000000  fp : 00000001
r10: 00000000  r9 : a6ffba8c  r8 : 00000000
r7 : a7fffad0  r6 : a6ffbb04  r5 : a6fdd5cc  r4 : a6fdd658
r3 : a7f18760  r2 : 00000000  r1 : 00000000  r0 : 00000000
Flags: nZCv  IRQs off  FIQs off  Mode SVC_32
[<a7f24700>] (simple_strtoul+0xc/0xc0) from [<a7f193f4>] (mci_set_probe+0xc94/0xd64)
[<a7f193f4>] (mci_set_probe+0xc94/0xd64) from [<a7f252cc>] (dev_set_param+0x74/0x8c)
[<a7f252cc>] (dev_set_param+0x74/0x8c) from [<a7f0606c>] (setenv+0x60/0xd8)
[<a7f0606c>] (setenv+0x60/0xd8) from [<a7f00820>] (set_local_var.isra.3+0x5c/0x7c)
[<a7f00820>] (set_local_var.isra.3+0x5c/0x7c) from [<a7f01a6c>] (run_list_real+0x494/0x934)
[<a7f01a6c>] (run_list_real+0x494/0x934) from [<a7f01400>] (parse_stream_outer+0x198/0x270)
[<a7f01400>] (parse_stream_outer+0x198/0x270) from [<a7f0209c>] (run_shell+0x3c/0x7c)
[<a7f0209c>] (run_shell+0x3c/0x7c) from [<a7f05494>] (execute_command+0x38/0x78)
[<a7f05494>] (execute_command+0x38/0x78) from [<a7f01db8>] (run_list_real+0x7e0/0x934)
[<a7f01db8>] (run_list_real+0x7e0/0x934) from [<a7f019ec>] (run_list_real+0x414/0x934)
[<a7f39578>] (unwind_backtrace+0x0/0xa8) from [<a7f24ed4>] (panic+0x28/0x3c)
[<a7f24ed4>] (panic+0x28/0x3c) from [<a7f39a3c>] (do_exception+0x10/0x14)
[<a7f39a3c>] (do_exception+0x10/0x14) from [<a7f39ac8>] (do_data_abort+0x2c/0x38)
[<a7f39ac8>] (do_data_abort+0x2c/0x38) from [<a7f39788>] (data_abort+0x48/0x60)

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
dtc: remove autogenerated modules.order

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>